Project Summary Metastatic prostate cancer is incurable. Defining the key contributors to metastatic progression within the primary tumor will have broad implications for clinically translatable targets for both the early identification of who is at risk for metastatic cancer and for the treatment of cancer. In patient based data, we have identified germline variants in the length of the aspartate (D) repeat domain of Asporin (ASPN) that are either associated with (D14) or are protective of (D13) metastatic development. We reported that opposed to being expressed in cancer cells, ASPN expression is highly enriched in cancer associated fibroblasts (CAFs), and its expression is significantly associated with local cancer aggressiveness as measured by Gleason grade. Consistent with patient-based data, in vivo models of tumor-stromal interactions, support that expression in the tumor microenvironment of ASPN D14 drives while ASPN D13 restricts metastatic development. Our data suggest that ASPN has potential clinical utility to better stratify disease aggressiveness for treatment decision making and may also be a potential therapeutic target due to its extracellular expression in the tumor microenvironment. Despite its high potential for clinical significance, the role of ASPN in local tumor aggressiveness or metastatic development has not been fully determined. Furthermore, the cellular and molecular mechanisms by which ASPN regulates metastatic progression are also incompletely defined. However, our preliminary data suggest that ASPN may regulate pluripotency and self-renewal of CAF progenitors, which may be a key mechanism for how ASPN regulates local progression and metastatic potential. Based on these new findings, we aim to identify the mechanisms by which ASPN regulates tumor progression. Our central hypothesis is that ASPN maintains CAF progenitors, and thereby ASPN potentiates CAF-induced invasion and progression to metastasis. We further hypothesize that the length of the D-repeat domain in ASPN affects its mechanistic and cellular functions and ultimately its impact on metastatic development. Our goals are 1) to determine the contribution of ASPN to primary tumor growth and metastatic development in autochthonous animal models, and to delineate the 2) cellular and 3) molecular based mechanisms by which ASPN D14 promotes and ASPN D13 restricts tumor progression and metastatic-invasion of prostate cancer. We propose to comprehensively examine the role of ASPN in local cancer aggressiveness and metastatic development using novel in vivo animal models, cutting edge in vitro assays, and patient-based data. Successful completion of these aims will provide critical information on how ASPN regulates metastatic development, and will therefore provide opportunities for translating these findings for prognosis and therapy.